Photocatalyst compositions and methods for making the same

ABSTRACT

Photocatalyst compositions may include a photocatalyst layer formed or disposed on the surface of a porous substrate. A metal may be disposed on the photocatalyst layer. If the metal is present predominantly at the surface of the photocatalyst layer, the metal can be utilized efficiently for photocatalytic reactions. The photocatalyst composition may be preferably formed by disposing the photocatalyst layer on the surface of the porous substrate and depositing the metal predominantly on the surface of the photocatalyst layer. Photocatalytic filter devices may include these photocatalyst compositions.

[0001] This application claims priority to Japanese Patent Application No. 2001-215910, filed Jul. 11, 2002, the contents of which are hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present teachings relate to compositions comprising a photocatalytic material (photocatalyst composition) and a metal and methods for making the same. Photocatalytic filter devices using these photocatalyst materials are also taught.

[0004] 2. Description of the Related Art

[0005] Metal oxides (e.g., semiconductor compounds) such as titanium dioxide generate electron-hole pairs (i.e., become polarized) when energy is applied to the metal oxide in excess of the bandgap (e.g., 3.2 eV for an anatase crystal) of the metal oxide. These electron-hole pairs may be utilized to provide photocatalytic effects. For example, Japanese Laid-Open Patent Publication No. 2001-38218 describes a photocatalytic filter in which a photocatalyst is disposed on the surface of a porous ceramic substrate.

[0006] In addition, if a metal (e.g., silver) is disposed on a substance that is functioning as a photocatalyst (e.g., fine TiO ₂ particles), photocatalytic efficiency can be increased. When the photocatalyst is polarized, the electron-hole pairs may have very short lifetimes. In this case, the electron-hole pairs may recombine and disappear before oxidizing or reducing an externally supplied substance. Thus, the photocatalyst may not exhibit photocatalytic activity. This disappearance (i.e., the reduction in number of electrons and holes) has been a significant factor in reducing the reaction efficiency of photocatalytic materials.

[0007] However, if a metal (e.g., silver) is disposed on the photocatalytic substance, the polarization is stabilized and therefore, photocatalytic efficiency can be increased. For example, Japanese Laid-Open Patent Publication No. 10-146531 describes a photocatalyst having fine metal particles in which the photocatalytic efficiency (photocatalytic activity) was improved by disposing ultra-fine metal particles on the surfaces of the fine titanium dioxide particles. Further, Japanese Laid-Open Patent Publication No. 10-33990 describes a silver-based catalyst in which silver and/or a silver compound was disposed on the surface of an alumina powder or another inorganic oxide. In this publication, the photocatalyst includes a substrate or support (e.g., a honeycomb structure) made of a ceramic material that has been coated with a silver-based catalyst.

[0008] As shown in FIG. 18, both the fine metal particle-photocatalyst described in Japanese Laid-Open Patent Publication No. 10-146531 and the silver-based catalyst described in Japanese Laid-Open Patent Publication No. 10-33990 include particles 90 that comprises metal particles 94 (e.g., silver), which have been uniformly deposited on substantially the entire surface of a photocatalytic particle 92, which is a semiconductor substance such as titanium dioxide. When a substrate or support (not shown) is coated with a slurry containing the metal-photocatalyst particles 90 and then fired, a coating layer 96 including the metal particles 94 is formed on the surface of the substrate, as shown in FIG. 19.

[0009] However, when the coating layer 96 is formed of such metal-photocatalyst particles 90, a substantial amount of the metal particles 94 are disposed between adjacent photocatalytic particles 92, as shown in FIG. 19. Therefore, the strength of the coating layer 96 shown in FIG. 19 tends to be less than the strength of a photocatalytic layer 98 formed of photocatalytic particles 92 that are free from metals, as shown in FIG. 20. Furthermore, the surface properties of the metal-photocatalyst particles 90 are very different from the surface properties of photocatalytic particles 92 that are free from the metal. Therefore sufficient consideration should be given to the conditions under which the coating layer 96 is formed in view of the type of metal particles 94 and the amount of the metal that is disposed on the photocatalytic particle 92.

[0010] In addition, the coating layer 96 contains metal particles 94 that are substantially uniformly disposed throughout the layer (i.e., both on the surface of the coating layer 96 and within the interior portion of the coating layer 96). However, under ordinary conditions for use, sufficient light can not reach the interior portion of the coating layer 96 in order to sufficiently activate the photocatalyst material. Therefore, the metal particles 94 that are disposed within the interior portion cannot effectively be utilized for photocatalytic purposes. In other words, the amount of the metal 94 that can be used to increase the photocatalytic activity of the photocatalyst particles 92 may be significantly less than the total amount of metal 94. Thus, metal utilization efficiency is low in the known photocatalytic compositions.

SUMMARY OF THE INVENTION

[0011] It is, accordingly, one object of the present teachings to provide photocatalyst compositions in which a photocatalyst and a metal are disposed on a substrate or support in a manner that enables the metal to be efficiently utilized. It is another object of the present teachings to provide photocatalytic filter devices that include such photocatalyst compositions. It is yet another object of the present teachings to provide methods for making the photocatalyst compositions.

[0012] In one aspect of the present teachings, photocatalyst compositions may include a porous substrate A photocatalyst layer may be disposed or formed on the surface of the porous substrate and the photocatalyst layer preferably substantially comprises a photocatalytic substance. Metal or metal particles are disposed predominantly on the surface of the photocatalyst layer.

[0013] Preferably, most or all the metal (metal particles) is present at a position within the photocatalyst layer that exhibits the strongest photocatalytic activity (e.g., the surface of the photocatalyst layer). In other words, most of the metal is disposed at a position that can improve or increase the photocatalytic activity of the photocatalyst material. As a result, the metal included in the photocatalyst composition can be effectively utilized.

[0014] In the present specification, the term “porous substrate” is intended to mean a substrate having pores or hole through which a fluid can pass, e.g., at atmospheric pressure or at a pressure higher than atmospheric pressure. For example, preferred porous substrates may have a three-dimensional structure, such as a honeycomb structure or a three-dimensional network. Particularly preferred porous substrates may have a three-dimensional network structure with pore sizes that allow light to permeate into the interior portion of the three-dimensional network structure.

[0015] Further, in the present specification, the term “hard porous substrate” is intended to mean a substrate that does not significantly deform (e.g., dissolve, expand, etc.) when the porous substrate contacts an aqueous solvent, such as water or a mixed solvent based on water, and substantially maintains the three-dimensional structure of the porous substrate in an aqueous environment. For example, ceramics having a three-dimensional network structure may be advantageously utilized as hard porous substrates.

[0016] In a representative embodiment of the present teachings, at least one metal (metal particles) of the photocatalyst composition may be selected from the group consisting of silver (Ag), gold (Au), platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), copper (Cu) and nickel (Ni). If one of these metals is disposed on the photocatalyst layer, the photocatalytic activity (efficiency) of the photocatalyst composition may be increased as compared to photocatalyst compositions that do not include such metals. The metal is preferably disposed on the photocatalyst layer in the form of fine metal particles having an average particle size of 100 nm or less. Such metal particles may significantly increase the photocatalytic efficiency (photocatalytic activity) of the photocatalyst material.

[0017] Optionally, photocatalyst compositions of the present teachings may exhibit a light transmissivity of at least 10% when the porous substrate (e.g., a substantially plate-shaped porous material) has a thickness of 5 mm. In photocatalyst compositions exhibiting such light transmission properties, sufficient light can permeate into (can be transmitted through) the pores of the porous substrate in order to efficiently utilize the metal that is present predominantly on the photocatalyst layer within the interior portion of the pores of the porous substrate, as well as at the surface of the photocatalyst layer.

[0018] In another aspect of the present teachings, photocatalytic filter devices are taught that may include at least one of the above-described photocatalyst compositions as a primary photocatalyst material. The present photocatalytic filter devices may be advantageously utilized, e.g., to purify a fluid by causing the fluid (e.g., a gas, such as air, or a liquid, such as water) to flow through the photocatalyst composition (filter portion). For example, organic substances may be removed from the fluid by decomposing the organic substances utilizing the photocatalytic effects of the photocatalyst composition.

[0019] In another aspect of the present teachings, methods are taught for making the above-described photocatalyst compositions. In one representative method, a porous substrate may be prepared (or purchased). Then, a photocatalyst layer made substantially of a photocatalytic material may be disposed or formed on the surface of the porous substrate. Thereafter, a metal (metal particles) may be disposed on the photocatalyst layer such that the metal is present predominantly at a surface of the photocatalyst layer. The metal (or a combination of metals) is preferably selected so as to increase or improve the photocatalytic activity of the photocatalyst material.

[0020] In one representative embodiment, a photocatalyst layer containing a photocatalytic substance (e.g., TiO₂, ZnO, WO₃, Cu₂O or a similar semiconductor compound) is disposed or formed on the surface of the porous substrate. Then, the metal is disposed on the surface of the photocatalyst layer. This representative method enables the metal to be disposed predominantly at the surface of the photocatalyst layer.

[0021] By disposed or depositing the metal on the surface of the photocatalyst layer after the photocatalyst layer has been formed from a photocatalytic substance (e.g., titanium dioxide), a denser photocatalyst layer can be formed as compared to known photocatalyst layers formed from an aggregate of metal-carrying particles. Thus, the photocatalyst layer of the present photocatalyst composition has excellent mechanical strength.

[0022] Optionally, the metal may be disposed or deposited on the photocatalyst layer by contacting the photocatalyst layer with a solution containing the metal and then irradiating the photocatalyst layer using light having a wavelength that can activate the photocatalytic substance. When light irradiation is performed in this manner, the metal (typically metal ions) in the solution is reduced by the photocatalyst effect of the photocatalytic substance contained in the photocatalyst layer. As a result, the reduced metal will be deposited on the surface of the photocatalyst layer. Accordingly, the metal disposed on the photocatalyst layer is present predominantly at the surface of the photocatalyst layer. As noted above, the predominantly present metal is preferably selected from at least one of Ag, Au, Pt, Pd, Ru, Rh, Cu and/or Ni.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a schematic view showing a representative structure of a ceramic substrate.

[0024]FIG. 2 is a cross-sectional view taken along line II-II shown in FIG. 1.

[0025]FIG. 3 is a schematic view showing the structure of a ceramic substrate that includes ceramic particles.

[0026]FIG. 4 is a cross-sectional view taken along line IV-IV shown in FIG. 3.

[0027]FIG. 5 is a schematic cross-sectional view showing a structure in which a photocatalyst layer is formed on the surface of the ceramic substrate shown in FIG. 2.

[0028]FIG. 6 is a schematic cross-sectional view showing a structure in which a photocatalyst layer is formed on the surface of the ceramic substrate shown in FIG. 4.

[0029]FIG. 7 is a schematic cross-sectional view showing a photocatalyst composition in which metal particles are present predominantly at the surface of a photocatalyst layer.

[0030]FIG. 8 shows a representative gas treatment apparatus.

[0031]FIG. 9 is a perspective view showing a photocatalytic filter that may be utilized as one component of the gas treatment apparatus shown in FIG. 8.

[0032]FIG. 10 is a plan view showing a photocatalyst module that may be utilized as one component of the gas treatment apparatus shown in FIG. 8.

[0033]FIG. 11 is a cross-sectional view taken along line XI shown in FIG. 10.

[0034]FIG. 12 shows a representative water treatment apparatus.

[0035]FIG. 13 is a perspective view showing a representative photocatalyst unit that may be utilized with the water treatment apparatus of FIG. 12.

[0036]FIG. 14 is a schematic cross-sectional view showing a photocatalyst treatment device that was used to evaluate the performance of photocatalyst compositions according to the present teachings.

[0037]FIG. 15 is a graph showing temporal changes in NO and NO₂ concentration of a discharged gas according to Example 3.

[0038]FIG. 16 is a graph showing temporal changes in SO₂ concentration of a discharged gas according to Example 4.

[0039]FIG. 17 is a graph showing temporal changes in methyl mercaptan concentration of a discharged gas according to Example 5.

[0040]FIG. 18 is a schematic cross-sectional view showing the structure of a known metal-carrying particle.

[0041]FIG. 19 is a schematic cross-sectional view showing a coating layer formed from known metal-carrying particles.

[0042]FIG. 20 is a schematic cross-sectional view showing a photocatalyst layer formed of photocatalyst particles on which a metal is not disposed.

DETAILED DESCRIPTION OF THE INVENTION

[0043] The porous substrate of the present photocatalyst compositions may comprise metal materials, inorganic materials and/or organic materials. The terms “porous substrate” and “porous support” will be utilized interchangeably throughout the present specification and the intended meaning of these two terms the same. If the photocatalyst composition will be utilized in an aqueous medium or environment, the porous substrate preferably does not significantly deform (e.g., dissolve, expand, etc.) when it contacts the aqueous medium.

[0044] Representative porous substrates or supports include non-woven or woven fabrics made of inorganic fibers such as metal fibers (stainless steel, aluminum, etc.), glass fibers, carbon fibers or the like, non-woven or woven fabrics made of resin fibers (e.g., polyurethanes, polyamides, polyesters, or polyolefins), foam substances made of such resin materials, and porous substances made of ceramic materials. If the porous substrate comprises or is substantially made of an organic material, the substrate surface may preferably be coated with a material having low photocatalyst reactivity, thereby serving as an undercoating layer, before forming the photocatalyst layer, which will be further described below.

[0045] Particularly preferred porous substrates include ceramics having a three-dimensional network structure (hereinafter “porous ceramic substrate”). The average diameter of the backbone of such a ceramic material is preferably between about 100-1000 μm. If the average diameter of the backbone of such porous ceramic substrates is at least 100 μm (and more preferably at least 200 μm), the porous ceramic substrates will have good manufacturing and handling properties, because these porous ceramic substrates will have suitable mechanical strength. However, if the average diameter of the backbone is greater than 1000 μm, it becomes difficult to appropriately balance the light transmission properties of the photocatalyst composition (which includes the porous ceramic substrate) with the surface area per unit volume of the photocatalyst composition.

[0046] A representative method for making appropriate porous ceramic substrates will be described as follows. First, a fine ceramic powder (e.g., one or two or more fine powders comprising alumina, silica, mullite and the like can be used), a binder serving as a binding material (e.g., one or more of organic binders such as dextrin, methyl cellulose, polyvinyl alcohol or the like, or inorganic binders such as clay, sodium silicate or the like can be used) and optionally water are mixed and stirred to prepare a slurry. This slurry serves as a precursor for forming the porous ceramic substrate. Then, the slurry is impregnated into an organic porous substance (e.g., a polyurethane foam substance) having a three-dimensional network structure. Thereafter, the slurry is dried and fired so that the organic porous substance is decomposed and the resulting fine ceramic powder is sintered. Thus, as shown in FIGS. 1 and 2, a porous ceramic substrate 71 having a three-dimensional structure comprising the sintered fine ceramic powder is obtained. As shown in FIG. 2, a pore or hole 78 is formed at positions in which the organic porous substance is decomposed.

[0047] Optionally, the porous ceramic substrate may include ceramic particles that are disposed on the surface of the backbone. In this case, the porous ceramic substrate will have a rough or uneven surface due to the ceramic particles. The rough surface may serve to anchor a photocatalyst layer, as will be further described below. Further, the rough surface effectively increases the surface area of the porous ceramic substrate. Therefore, the amount of photocatalyst per unit volume can be increased. In addition, the rough surface will increase the surface area per unit volume of the photocatalyst composition. For ceramic particles, an average particle diameter of at least 1 μm and at most 100 μm is preferable, and an average particle diameter of at least 10 μm and at most 50 μm is more preferable. An average particle diameter of less than 1 μm will reduce the effect of increasing the roughness to the porous ceramic substrate. However, if the average particle diameter is greater than 100 μm, it is difficult to stably dispose the ceramic particles on the surface of the porous ceramic substrate.

[0048] Such porous ceramic substrate having ceramic particles disposed on the surface can be produced in the following manner. An organic porous substance may be impregnated with the slurry according to the above-described method for making a porous ceramic substrate. Then, ceramic particles (one or two or more particles comprising alumina, silica mullite and the like can be used) are scattered and attached to the organic porous substance that has been wetted with the slurry. Thereafter, the ceramic substrate is dried and fired. As shown in FIGS. 3 and 4, porous ceramic substrate 71 may include ceramic particles 72 that are disposed along the surface of backbone 77. In portions of FIG. 3, the representation of the ceramic particles 72 has been omitted so that the backbone 77 is exposed for purposes of illustration.

[0049] The main component of the photocatalyst layer disposed on the surface (i.e., both the inner and outer surfaces of the pores of the structure) of the porous substrate preferably contains a material exhibiting photocatalyst activity (e.g., a metal oxide-based photocatalytic substance). Such photocatalyst materials may include one, two or more compounds, such as titanium oxide, tungsten oxide, zinc oxide, vanadium oxide, zirconium oxide and/or other metal oxides exhibiting photocatalyst activity. Titanium oxide is a particularly preferred material. Further, the photocatalyst layer can be formed by preparing a slurry containing ultra-fine particles of a photocatalytic material as the main component and an organic or inorganic binder, impregnating a porous substrate with the slurry, and then drying and firing the porous substrate.

[0050]FIG. 5 schematically shows a photocatalyst layer 76 disposed or formed on the porous ceramic substrate 71 shown in FIG. 2. FIG. 6 schematically shows a photocatalyst layer 76 disposed or formed on the porous ceramic substrate 71 shown in FIG. 4.

[0051] As noted above, the porous substrate with the photocatalyst layer preferably transmits at least 10% of light through a thickness of 5 mm, more preferably transmits at least 20%, and even more preferably transmits at least 30%. Thus, the present photocatalyst compositions may exhibit a relatively high light transmissivity (e.g., light transmission of at least 10%, more preferably at least 20% and even more preferably at least 30%). There are no particular limitations regarding the upper limit of the light transmission. However, if the light transmission is greater than 50%, handling ease of the substrate having a photocatalyst and/or the mechanical strength retention may be disadvantageously decreased.

[0052] As noted above, the metal (metal particles) disposed on the photocatalyst layer may be one or a combination of two or more transition metals. Preferred examples include Ag, Au, Pt, Pd, Ru, Rh, Cu and Ni. Precious metals such as Au, Ag, Pt, and Pd are more preferable, and Ag is even more preferable. These metals can be disposed in a stable state for a long time, and their catalytic activity (performance of improving photocatalytic efficiency) can be maintained for a relatively long time. As a result, the performance of the photocatalyst composition (photocatalytic efficiency) can be maintained over a long time.

[0053] Furthermore, metals exhibiting antimicrobial properties, such as Ag and Cu, may preferably be used in the present photocatalyst composition. If one or more such metals are utilized, the growth of microorganisms can be suppressed on the surface of the photocatalyst composition even when the photocatalyst composition is not being irradiated (i.e., during the period when electron-hole pairs are not being generated and polarization is not present). Therefore, such photocatalyst compositions exhibiting high antimicrobial properties can prevent microorganisms (e.g., bacteria) from growing in or on the photocatalyst composition even without continuously irradiating the photocatalyst composition.

[0054] As noted above, the metal or metal particles preferably are substantially all disposed on the surface of the photocatalyst layer. Thus, at least 70 wt % (more preferably at least 80 wt % and even more preferably at least 95 wt %) of the metal is present on and near the surface of the photocatalyst layer. More preferably, substantially all of the metal is present at the surface of the photocatalyst layer and substantially no metal is contained within the interior portion of the photocatalyst layer. Naturally, if metal is buried deep within the interior portion, the amount of metal that does not contribute to the photocatalytic activity is less than the total amount of metal contained within the photocatalytic composition. Thus, by disposing substantially all the metal in a position in which the substantially all the metal contributes to the photocatalytic activity of the photocatalytic composition, the utilization efficiency of the metal included in the photocatalyst composition can be high.

[0055] The metal is preferably disposed on the photocatalyst layer in the form of metal particles. The average metal particle diameter is preferably 1000 nm or less, more preferably 100 nm or less, and even more preferably 30 nm or less. When the average particle diameter is larger than 1000 nm, the photocatalytic effects of the metal are reduced. There are no particular limitations regarding the lower limit of the average diameter of a metal particle. However, it is noted that metal particles having an average diameter of less than 1 nm are difficult to manufacture and utilize with current manufacturing techniques.

[0056] At least 70% by number (more preferably at least 80% and particularly preferably at least 95%) of the metal particles preferably are present at or near the surface of the photocatalyst layer. More preferably, substantially all of the metal particles are present at the surface of the photocatalyst layer (i.e., almost no metal particles are present within the interior portion of the photocatalyst layer). The ratio (uneven distribution) of metal that is present at the surface of the photocatalyst layer and the average particle diameter of this metal can be easily and reliably investigated, e.g., by utilizing an electron microscope, such as a TEM or a similar device.

[0057] In another embodiment of the present teachings, the ratio of the amount of the metal disposed on the photocatalyst layer with respect to the amount of the photocatalytic substance contained within the photocatalyst layer is preferably in the range from 0.001 wt % to 20 wt %, and more preferably in the range from 0.005 wt % to 5 wt %. When the content of the metal is less than these ranges, the photocatalytic activity is not significantly increased or improved. On the other hand, when the content of the metal is higher than these ranges, the metal may reflect or absorb the externally supplied light. As a result, the amount of light that reaches the photocatalyst layer may be reduced. In other words, photocatalytic activity of the metal may be reduced or eliminated, if too much metal is utilized. Naturally, the amount of metal that is utilized is preferably minimized in order to reduce production costs of the photocatalyst material.

[0058] No particular restrictions are placed on the methods for disposing or depositing the metal on the surface of the photocatalyst layer. For example, the metal can be disposed predominantly at the surface of the photocatalyst layer using known impregnation methods. In one representative, but not limiting example, a substrate having a photocatalyst layer may be immersed into an aqueous solution containing a salt (e.g., nitrate, chloride, sulfate and carbonate) of a metal (e.g., silver). The substrate may then be dried and subjected to a reduction treatment.

[0059] In one particularly preferred method, the photocatalyst layer may be contacted with a solution containing the metal (e.g., a metal salt). Then, the photocatalyst layer and the solution are irradiated with light having a wavelength that will activate the photocatalytic substance in the photocatalyst layer. For example, if the photocatalyst is TiO₂, UV rays having a wavelength of preferably 380 nm or less may be utilized. Hereinafter, this method will be referred to as a “light irradiation technique” or an “optical electrodeposition technique”. As a further example, a substrate having a photocatalyst layer may be immersed into a solution (e.g., an aqueous solvent solution, and typically an aqueous solution) containing metal ions and then the substrate having the photocatalyst layer is irradiated with light while being disposed in the aqueous solution. Thus, the metal ions are then reduced and precipitated on the surface of the photocatalyst layer. Consequently, the precipitated metal can be disposed on the surface of the photocatalyst layer in the form of fine metal particles having an average particle diameter of about 1 to 10 nm.

[0060] The present light irradiation techniques enable fine metal particles having an average particle diameter suitable for improving photocatalytic activity (increasing the reaction efficiency) to be disposed substantially uniformly along the surface of the photocatalyst layer. FIG. 7 schematically shows the photocatalyst composition having metal particles 80 that are disposed predominantly on the surface of the photocatalyst layer 76.

[0061] The light source for deposition of the metal preferably emits primarily light having a wavelength (or wavelengths) that enable(s) the photocatalyst to function appropriately. Naturally, different types of photocatalyst may require different wavelengths. For example, a fluorescent lamp, such as a black light, or an ultraviolet lamp, such as an extra-high pressure mercury lamp or an extra-low pressure mercury lamp, may be utilized for this process. When titanium dioxide is used as the photocatalyst, an ultraviolet lamp is preferably utilized that emits ultraviolet rays having a wavelength of at least 300 nm and at most 420 nm (e.g., ultraviolet rays having a peak of at least 360 nm and at most 380 nm).

[0062] In the present light irradiation techniques, the metal is deposited due to photocatalytic activity at the portion of the photocatalyst layer that is externally irradiated. Naturally, the portion that is irradiated at this time will likely correspond to the portion that is irradiated when the resulting photocatalyst composition is in use (at the time of light irradiation). In other words, the portion is irradiated at which the photocatalyst functions well and increases the photocatalytic efficiency due to the metal. Therefore, the present light irradiation techniques can efficiently dispose the metal on the photocatalyst layer. Consequently, the utilization efficiency of the metal can be increased and a relatively small amount of the metal can provide a relatively large effect. Furthermore, the cost of the raw material for manufacturing the photocatalyst composition can be decreased. In particular, when precious metals are used as the metal, a significant reduction of the cost can be expected.

[0063] During light irradiation, the photocatalytic material contained in the photocatalyst layer can reduce metal ions in the solution. Therefore, it is not necessary to add an additional reductant to the solution. However, a reductant may be added to a solution containing metal ions in order to assist the reduction of the metal ions by photocatalysis.

[0064] By either the irradiation technique or the impregnation technique, the substrate having the photocatalyst layer is brought into contact with the solution (e.g., an aqueous solvent solution) containing the metal. The substrate having the photocatalyst layer may be a hard porous substrate, so that the porous substrate is not substantially deformed by contacting the aqueous solution. Therefore, in this method, even if the substrate having the photocatalyst layer contacts the aqueous solution in order to deposit the metal on the surface of the photocatalyst layer, the photocatalyst layer is not significantly damaged (i.e., the photocatalyst layer will not peel off or dislodge from the hard porous support).

[0065] As noted above, the present photocatalyst compositions preferably exhibit a light transmissivity of at least 10% at a thickness of 5 mm. This light ratio indicates the amount of light that is transmitted through a structure 5 mm deep from the incident surface with respect to the incident light. More preferably, the light transmissivity is at least 20% and even more preferably at least 30%. If the photocatalyst composition exhibits a light transmissivity of at least 10%, light can be easily transmitted into the interior portion of the photocatalyst composition. Therefore, most of the photocatalyst and the metal included in the photocatalyst composition can be effectively utilized.

[0066] The above-described light transmissivity ranges can be achieved by satisfying at least one of the following three conditions (preferably two or more, and more preferably all three conditions): (1) the porosity is at least 65% and at most 99% (typically, at least 70% and at most 95%), (2) the bulk density is at least 0.05 g/cm³ and at most 0.60 g/cm³ (typically, at least 0.15 g/cm³ and at most 0.50 g/cm³); and (3) the number of cells is at least 10 cells per 25 mm (10/25 cm) and at most 30 cells per 25 mm (30/25 cm). Herein, “the number of cells (X/25 mm)” indicates the number of cells that intersect a 25 mm straight line that is drawn along the surface or the cross-section of a porous ceramic substrate. Naturally, as this number increases, the pore sizes within the porous ceramic substrate are reduced. For example, suitable light transmissivity can be achieved by using a ceramic substrate having a three-dimensional network structure that satisfies at least one of the conditions (1), (2) and (3) (preferably two or more, and more preferably all three conditions).

[0067] A photocatalyst composition having a porosity of 99% or less, a bulk density of 0.05 g/cm³ or more and a number of cells of 30/25 mm or less will exhibit suitable mechanical strength. Therefore, the photocatalyst will exhibit suitable manufacturing and handling properties. However, if the photocatalyst composition has a porosity of less than 65%, a bulk density of more than 0.60 g/cm³ and/or a number of cells of less than 10/25 mm, only a small quantity of light may reach the interior portion of the photocatalyst composition. In addition, if such a photocatalyst composition is used as a filter, the pressure loss may be substantial when a fluid containing a substance to be treated (fluid to be treated) is passed through the photocatalyst.

[0068] Representative photocatalytic filter devices may include a filter that primarily contains one or more of the above-described photocatalyst compositions. The photocatalytic filter device optionally may include a supporting frame for maintaining the form of the photocatalyst composition. Further, an attaching member may be utilized to attach the photocatalyst composition to a predetermined position of the supporting frame. A light source may be provided for irradiating the photocatalyst composition. Further, a fluid passage defining member may be provided for introducing a fluid into the photocatalyst composition and a fan or pump may facilitate efficient passage of the fluid through the photocatalyst composition. If the filter device is used for treating exhaust containing a large amount of oil components (e.g., oil particles, oil smoke, etc.) such as exhaust generated by cooking, a de-oiling filter may be provided upstream from the photocatalyst composition in order to filter out oil components contained within the exhaust.

[0069]FIG. 8 shows a representative gas treatment apparatus, which is an example of such a photocatalytic filter device. Gas treatment apparatus 100 may include a light treatment portion 110 having a photocatalytic filter (photocatalyst composition) 62 and a gas supply device 120 for supplying a gas 150 that will be treated (e.g., indoor air) to the light treatment portion 110. The light treatment portion 110 may include a gas passage (fluid passage defining member) 112 that communicates with the downstream portion of the gas supply device 120. A prefilter 114 and a photocatalyst module 61 also may be disposed within the gas passage 112. The prefilter 114 is preferably made of non-woven fabrics or a similar substance and serves to remove (filter) contaminants (e.g., dust) contained within the gas 150 that will be treated.

[0070] As shown in FIGS. 10 and 11, the photocatalyst module 61 may include a plurality of (e.g., 12) photocatalytic filters 62 and a light source 63 for irradiating the photocatalytic filters 62. The photocatalytic filters 62 may be disposed so as to oppose each other with the light source 63 interposed therebetween. A frame 64 may integrally support each of these components.

[0071] As shown in FIG. 9, the photocatalytic filters 62 may each be defined in the form of a flat plate. The photocatalytic filter 62 preferably has a structure in which a photocatalyst layer 76 is formed, as shown in FIG. 7, on the surface of the porous ceramic substrate 71 having a three-dimensional structure as shown in FIG. 1. Further, the metal particles 80 are preferably disposed predominantly on the surface of the photocatalyst layer 76. The photocatalyst layer 76 may primarily contain titanium dioxide as the active photocatalyst component. The metal particles 80 may be silver particles.

[0072] Referring back to FIGS. 10 and 11, the frame 64 may comprise an anti-corrosion metal, such as stainless steel, in the form of a box and may include openings on opposing sides. A plurality of (e.g., 6) photocatalytic filters 62 may be arranged within the same plane, and may be supported by a lattice frame 68 in order to form a flat plate-like filter unit 69. Two filter units 69 may be attached to the openings provided on the opposite sides of the frame 64. A plurality of light sources 63 held by the frame 64 may be disposed in parallel to each other between the filter units 69. In the photocatalyst module 61 having this structure, the photocatalytic filters 62 are exposed to the outside from the openings on the opposite sides of the frame 64. The gas that will be treated passes through the photocatalyst module 61 via the two layered photocatalytic filters 62. In this embodiment, four ultraviolet lamps are used as the light sources 63.

[0073] The gas supply device 120 shown in FIG. 8 may include a fan (not shown) disposed within a casing 122. When the fan is driven, the gas 150 can be drawn in from an inlet 124 of the casing 122. The drawn gas 150 then flows from an outlet 126 to the gas passage 112 (light treatment portion 110). Thereafter, the gas 150 passes through the prefilter 114 and then through the photocatalytic filters 62 disposed within the photocatalyst module 61. Thus, the photocatalytic filters 62 capture any contaminants that remain within the gas 150. Furthermore, odorous components contained within the gas 150 will contact the photocatalyst layer 76 disposed on the surface of the photocatalytic filters 62. The contaminants and odorous components are then photolyzed due the photocatalytic activity of titanium dioxide, which may be the main component of the photocatalyst layer 76. Thus, the gas 150 is converted into purified air 152 and is discharged to the outside.

[0074] As shown in FIG. 8, a post treatment unit 116 may be provided downstream from the photocatalyst module 61. The post treatment unit 116 may preferably include a device that can purify (or deodorize) the gas 150 by a process other than photocatalytic activity. For example, a filter having an adsorption function using activated carbon can be used as the post treatment unit 116.

[0075] In another embodiment of the present teachings, FIG. 12 shows a water treatment apparatus 200, which is another representative photocatalytic filter device according to the present teachings. The water treatment apparatus 200 may include a light treatment portion 220 and an ozone supply portion 240. The light treatment portion 220 may include a treatment bath 222 made of quartz for storing water 250 that will be treated. The photocatalytic filter 62 may be accommodated within the treatment bath 222. Further, an ultraviolet lamp 224 may be disposed to the side of the treatment bath 222. A power source 226 is connected to the ultraviolet lamp 224. When the ultraviolet lamp 224 is turned on, the photocatalytic filter 62 is irradiated with ultraviolet rays via the side wall of the treatment bath 222. The photocatalytic filter 62 may have a flat plate-like shape and may be disposed so as to face the side wall (surface that is irradiated with ultraviolet rays) of the treatment bath 222.

[0076] The ozone supply portion 240 may include an ozone generator 242. Dry air 252 is first supplied to the ozone generator 242 in order to increase the ozone content and then is supplied to the treatment bath 222 via an ozone supply passage 244. An air diffusing tube 246 may be connected to the terminal end of the ozone supply passage 244. The air difusing tube 246 may be submerged in the water 250 that will be treated, which is disposed within the treatment bath 222. The diffusing tube 246 preferably generates small bubbles 254 containing ozone that pass through the water 250 that will be treated. A flow meter 248 may be disposed within the ozone supply passage 244 in order to monitor of the flow rate of the gas supplied from the ozone generator 242.

[0077] The water 250 that will be treated may contain contaminants (e.g., organic substances such as proteins, amino acids, and/or saccharides) and is disposed within the treatment bath 222 in which the photocatalytic filter 62 has been accommodated. The ultraviolet lamp 224 is turned ON in order to irradiate the photocatalytic filter 62 with ultraviolet rays. As a result, the contaminants in the water 250 will be photolyzed due to the photocatalytic activity of the titanium dioxide, which preferably may be the primary component of the photocatalyst layer. Thus, the treated water 250 can be purified. As indicated above, small bubbles 254 containing ozone are preferably injected into the water 250 via the air diffusing tube 246, because purification can be increased by combining the photocatalytic activity and contaminant degradation (ozonolysis) using ozone. The above-described water treatment apparatus 200 is particularly suitable for purifying water 250 that has a relatively high transparency.

[0078]FIG. 13 shows a representative photocatalyst unit 300 that can be utilized within another water treatment apparatus. The photocatalyst unit 300 may include a water passage tube 312 through which the water 250 to be treated will flow. A plurality of disk-shaped photocatalytic filters 62 may be spaced from each other within the water passage tube 312. The photocatalytic filters 62 may preferably be disposed perpendicular to the longitudinal axis of the water passage tube 312. In addition, a plurality of (e.g., four in this example) ultraviolet lamps 314 may be accommodated within the water passage tube 312. The ultraviolet lamps 314 may extend through the plurality of photocatalytic filters 62 along the longitudinal axis of the water passage tube 312. The ultraviolet lamps 314 are connected to a source power (not shown). When the ultraviolet lamps 314 are turned ON, the water 250 is supplied to the water passage tube 312 and passes through the photocatalytic filters 62, and any contaminants in the water 250 will photolyzed, thereby purifying the water 250.

[0079] The photocatalyst unit 300 can be also used as a component of a gas treatment apparatus. In this case, a gas that will be treated is supplied to the passage tube 312.

[0080] Each of the additional features and method steps disclosed above and below may be utilized separately or in conjunction with other features and method steps to provide improved photocatalytic compositions and methods for making and using the same. Detailed representative examples of the present teachings, which examples will be described below, utilize many of these additional features and method steps in conjunction. However, this detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Therefore, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the present teachings in the broadest sense, and are instead taught merely to particularly describe representative and preferred embodiments of the present teachings, which will be explained below in further detail with reference to the figures. Of course, features and steps described in this specification may be combined in ways that are not specifically enumerated in order to obtain other usual and novel embodiments of the present teachings and the present inventors contemplate such additional combinations.

EXAMPLE 1 Manufacture of a Photocatalyst Composition (1)

[0081] 446.5 g of fine ceramic powder (fine alumina powder), 16.0 g of talc, 36.5 g of Kibushi clay (a type of sedimentary clay), 155 g of water and 12.5 g of a dispersant were put into a 2 liter pot mill made of polyethylene. Further, alumina balls having a diameter of 10 mm were put up to about ⅓ of the pot mill and the mixture was stirred for 5 hours. Then, 127.1 g of an organic binder (product name “CERAMO TB-01” manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) were added to the mixture in the pot mill and stirred for 20 hours. This solution was utilized as a slurry for forming a porous ceramic substrate.

[0082] An organic porous substance having a three-dimensional network structure (polyurethane foam in this example) was impregnated with the slurry. Then, the polyurethane foam (product name “MF-13” manufactured by INOAC Corp.) was lifted out of the slurry and excess slurry was removed by pressing with a roller. The slurry lodged within the voids of the polyurethane foam was blown out with a spray so as to eliminate clogging of the voids. The polyurethane foam containing the slurry as dried at 70° C. for 24 hours and then fired at 1600° C. for one hour. This firing decomposed the polyurethane foam and sintered the alumina fine powder contained in the slurry. As a result, a porous ceramic substrate having a three-dimensional network structure was produced.

[0083] The porous ceramic substrate was immersed in a photocatalyst slurry (product name “STS-01” manufactured by Ishihara Sangyo Kaisha, Ltd.), excess slurry was removed by spraying, and dried at 200° C. Thus, a photocatalyst layer containing a photocatalyst (titanium dioxide) as the main component was formed on the surface of the porous ceramic substrate. The photocatalyst slurry used in this example contained fine particles of anatase-type titanium dioxide (photocatalyst) that were mono-dispersed in an aqueous medium.

[0084] Next, fine silver particles were deposited on the surface of the photocatalyst layer. More specifically, an aqueous silver nitrate solution (silver nitrate concentration of 9.3 mmol/m³ (9.3×10⁻³ mmol/liter)) was placed in a quartz cell and the substrate having the photocatalyst layer was immersed in the quartz cell. Then, the pH of the aqueous silver nitrate solution was adjusted to about pH 6.3 using a 0.4 normal(M) KOH aqueous solution and the aqueous solution was stirred for 30 minutes. Thereafter, oxygen in the quartz cell was removed by bubbling nitrogen through the aqueous solution. Then, the substrate having the photocatalyst layer in the quartz cell was irradiated with ultraviolet light having a wavelength of 365 nm for one hour using a 500 W high-pressure mercury lamp. Thereafter, the substrate having the photocatalyst layer was lifted out of the aqueous silver nitrate solution, washed, and then dried at 110° C. for 3 hours in a drying oven. As a result, a photocatalyst composition (sample 1) was produced with the fine silver particles disposed on the surface of the photocatalyst layer.

EXAMPLE 2 Manufacture of a Photocatalyst Composition (2)

[0085] A photocatalyst composition (sample 2) was produced in the same manner as in Example 1 except for the following difference. Specifically, the aqueous silver nitrate solution having a silver nitrate concentration of 9.3 mmol/m³ was replaced with an aqueous silver nitrate solution having a silver nitrate concentration of 46 mmol/m³ (4.6×10⁻² mmol/liter). The amount of the fine silver particles disposed on the substrate having the photocatalyst layer was thus controlled by the concentration of the silver nitrate.

[0086] Table 1 shows the silver nitrate concentration of the aqueous silver nitrate solution used in Examples 1 and 2, the amount (average of the entire photocatalyst composition) of fine silver particles per unit volume of the resulting photocatalyst composition (samples 1 and 2), and the ratio (in weight) of the amount of the disposed fine silver particles to the weight of the photocatalyst (titanium dioxide). The amount of the disposed fine silver particles and the ratio thereof with respect to the weight of the photocatalyst were calculated based upon measured values obtained by measuring the amount of Ag that remained in the aqueous silver nitrate solution after the substrate having the photocatalyst layer was lifted out. This measurement was performed using an inductive coupling plasma emission analysis apparatus (ICP-AES). TABLE 1 Silver nitrate Amount of Ag concentration deposited Ag/TiO₂ (mmol/m³) (mg/cm³) (wt %) Example 1 9.3 5.0 × 10⁻³ 0.014 Example 2 46 2.5 × 10⁻² 0.072

[0087] Observation using an electron microscope confirmed that substantially all fine silver particles were deposited on the surface of the photocatalyst layer in both samples 1 and 2. The average particle diameter of the fine silver particles was 3 nm.

[0088] Further, both samples 1 and 2 exhibited light transmissivity of 30% or more at a thickness of 5 mm. The light transmissivity of each sample was measured according the following process. A black light (product name “FL10BLB” manufactured by Toshiba Lighting & Technology Corporation providing wavelengths of 300 to 420 nm and a peak wavelength 360 nm) was placed 7 cm from the surface of the samples. An ultraviolet intensity meter (product name “UM-10” manufactured by Minolta Co., Ltd.) was placed in contact with the back surface of the sample and the intensity of the ultraviolet rays that were transmitted through the samples was measured. Light transmissivity was calculated based upon the ratio of the intensity when the sample was interposed between the black light and the ultraviolet intensity meter to the intensity when the sample was not interposed between them, as follows:

Light transmissivity (%)={(intensity measured when the sample is interposed)/(intensity measured when the sample was not interposed)}×100

[0089] Both samples 1 and 2 satisfied the following three conditions: (1) the porosity was at least 65% and at most 95%; (2) the bulk density was at least 0.15 glcm³ and at most 0.60 g/cm³; and (3) the number of cells was at least 10/25 mm and at most 30/25 mm. Herein, the porosity was calculated from the volume, the mass and the density of the sample. The bulk density was calculated from the volume and the mass of the sample. Further, the number of cells was measured by observation using an optical microscope.

EXAMPLE 3 Evaluation of Purification Performance of the Photocatalyst Composition (1)-NO Removal Performance

[0090] As shown in FIG. 14, a photocatalyst treatment device 40 was prepared with a main member 42, a window plate 45 and a black light 50 as a light source. An inlet 43 and an outlet 44 were defined on the side faces of the main member 42. The upper opening of the main member 42 was closed by the window plate 45 made of quartz glass. Thus, an interior passage 46 leading from the inlet 43 to the outlet 44 was defined within the photocatalyst treatment device 40. A height adjusting plate 48 having a thickness of 10 mm was set on the bottom surface of the interior passage 46 and a sample (photocatalyst composition) 49 was mounted thereon when measurements were performed. The black light 50 was disposed above the window plate 45. Nitrogen monoxide gas having a NO concentration of 1.0 ppm (diluted with air) and a relative humidity of 50% at 25° C. was supplied from the inlet 43 at a rate of 3 liters/min. Gas (discharged gas) exhausted from the outlet 44 was introduced into a chemiluminescence NO_(x) meter (not shown) in order to measure the NO and NO₂ concentrations.

[0091] The measurements were performed as follows. The sample 49 having a length of 50 mm, a width of 50 mm and a thickness of 13 mm was mounted on the height adjusting plate 48. The samples were the photocatalyst composition (sample 1) produced in Example 1 and the photocatalyst composition (sample 2) produced in Example 2. For comparison, the substrate having the photocatalyst layer obtained in the process of producing the photocatalyst composition in Example 1 (comparative sample 1: silver is not disposed on the photocatalyst layer) was also evaluated.

[0092] First, nitrogen monoxide gas was allowed to flow through the interior passage 46 for 10 minutes, and it was confirmed that the NO concentration of the discharged gas reached 0.9 ppm or more. Thereafter, the black light 50 was turned ON and the NO concentration and the NO₂ concentration of the discharged gas were measured over time. FIG. 15 shows the results of this measurement. As seen in FIG. 15, in comparative sample 1 in which silver particles are not included, after 10 minutes had passed from turning ON the black light, the NO concentration increased with time. This result indicates that the NO removal performance of the comparative sample 1 was reduced as time elapsed. On the other hand, in samples 1 and 2, in which silver particles were included, the NO concentration of the discharge gas remained low (0.6 ppm or less), regardless of the amount time that had passed after the black light was turned ON. This result indicates that the NO removal performance of the samples was maintained throughout the measurement period. Further, less NO₂ was produced (NO₂ concentration of the discharged gas) in samples 1 and 2 than in the comparative sample 1. This result indicates that in samples 1 and 2, NO is efficiently converted to NO₃ ⁻(and the generation of NO₂ was suppressed).

[0093] The amount of NO fixed to the photocatalyst composition in the form of nitrate ions (NO₃ ⁻) was obtained from the difference between the NO amount supplied from the inlet 43 and the total amount of NO and NO₂ discharged from the outlet 44 during measurement. Based upon this information, the ratio (NO₃ ⁻fixation ratio) of the NO fixed to the photocatalyst composition with respect to the supplied NO was calculated. Table 2 shows these results. The NO₃ ⁻fixation ratio of samples 1 and 2 was at least 15 times greater than the NO₃ fixation ratio of comparative sample 1. TABLE 2 NO₃ ⁻fixation ratio (%) Sample 1 26.9 Sample 2 19.6 Comparative Sample 1 1.3

EXAMPLE 4 Evaluation of Purification Performance of the Photocatalyst Composition (2)-SO₂ Removal Performance

[0094] The nitrogen monoxide gas that was used in Example 3 was replaced by sulfur dioxide gas having a SO₂ concentration of 1.0 ppm (diluted with air) and a relative humidity of 50% at 25° C. and was supplied at a rate of 3 liters/min. The discharged gas from the outlet 44 was introduced into an ultraviolet spectrophotometric SO₂ meter. Other aspects of Example 4 are the same as Example 3.

[0095] A sample was mounted on the interior passage 46 and sulfur dioxide gas was allowed to flow through the sample for 2 hours. Thereafter, the black light 50 was turned ON for one hour and then was turned off. In this example, the SO₂ concentration of the discharged gas was also measured over time during a period when the black light 50 was not turned ON, and the SO₂ adsorption ability of the photocatalyst composition was investigated. FIG. 16 shows the results of the measurement. As shown in FIG. 16, before the black light was turned ON, in comparative sample 1, the SO₂ concentration of the discharged gas gradually increased. This result indicates that the SO₂ removal performance (adsorption ability) decreased as the time elapsed. On the other hand, in samples 1 and 2, the SO₂ concentration of the discharge gas was maintained low, and SO₂ purification performance (adsorption performance) was maintained.

[0096] The amount of SO₂ adsorbed by the photocatalyst composition was calculated based upon the difference between the amount of SO₂ supplied from the inlet 43 and the amount of SO₂ discharged from the outlet 44 during the measurement period. Thereafter, the ratio (SO₂ adsorption ratio) of the SO₂ adsorbed by the photocatalyst composition with respect to the supplied SO₂ was calculated. Table 3 shows these results. Table 3 indicates that the SO₂ purification performances of samples 1 and 2 are better than comparative sample 1 TABLE 3 SO₂ adsorption ratio (%) Sample 1 43.1 Sample 2 41.7 Comparative Sample 1 37.7

EXAMPLE 5 Evaluation of Purification Performance of the Photocatalyst Composition (3)-Methyl Mercaptan Removal Performance

[0097] The nitrogen monoxide gas that was used in Example 3 was replaced with methyl mercaptan gas having a methyl mercaptan concentration of 1.0 ppm (diluted with air) and a relative humidity of 50% at 25° C., and was supplied at a rate of 3 liters/min. The discharged gas from the outlet 44 was collected in a fluoroplastic gas bag, and the methyl mercaptan concentration was measured with a gas detecting tube. Other aspects of Example 5 were the same as Example 3.

[0098] A sample was mounted on the interior passage 46 and methyl mercaptan gas was allowed to flow through it for 10 minutes. It was confirmed that the methyl mercaptan concentration of the discharged gas reached about 1.0 ppm. Thereafter, the black light 50 was turned ON and the methyl mercaptan concentration of the discharged gas was measured over time. FIG. 17 shows the results of this measurement. As shown in FIG. 17, in comparative sample 1 in which silver is not contained, the methyl mercaptan concentration of the discharged gas was equal to the supplied gas, which indicates that the methyl mercaptan is not significantly decomposed. On the other hand, in samples 1 and 2, the methyl mercaptan concentration of the discharge gas was lower (about 0.8 ppm) than the supplied gas, which indicates that the methyl mercaptan was decomposed.

[0099] The amount of methyl mercaptan removed by the photocatalyst composition was calculated based upon the difference between the methyl mercaptan amount supplied from the inlet 43 and the methyl mercaptan amount discharged from the outlet 44 during the measurement period. Thereafter, the ratio (methyl mercaptan removal ratio) of the methyl mercaptan removed by the photocatalyst composition with respect to the supplied methyl mercaptan was calculated. Table 4 shows these results. TABLE 4 Methyl mercaptan removal ratio (%) Sample 1 15 Sample 2 22 Comparative Sample 1  0

EXAMPLE 6 Evaluation of Antibacterial Properties (1)

[0100] An E. Coli suspension (strain: E. Coli K-12, the number of cells (in 0.1 ml): 6×10⁸) in an amount of 0.1 ml was added to sample 1 and sample 1 was irradiated with ultraviolet light at an intensity of 1200 μW/cm² (360 nm) for 30 minutes. Then, sample 1 was immersed in a brain heart infusion (BHI) medium and the medium was cultured at 37° C. in an incubator for 16 hours. The medium that had been subjected to culturing was collected and applied to a BHI agar medium in a petri dish. Thereafter, this petri dish was cultured at 37° C. in an incubator for 10 hours. After the culturing ended, no colonies of E. Coli were observed in the petri dish.

EXAMPLE 7 Evaluation of Antibacterial Properties (2)

[0101] The antibacterial properties were evaluated in the same manner as in Example 6, except for the following difference. An E. Coli suspension was added to sample 1 in the same manner as in Example 6. Then, instead of irradiating sample 1 with ultraviolet light, sample 1 was shielded from light using aluminum foil. After the culturing ended, no colonies of E. Coli were observed in the petri dish.

COMPARATIVE EXAMPLE 1 Evaluation of Antibacterial Properties (3)

[0102] The antibacterial properties were evaluated in the same manner as in Example 6 (i.e., with ultraviolet irradiation for 30 minutes), except that sample 1 was replaced with comparative sample 1 (silver was not contained). After the culturing ended, no colonies of E. Coli were observed in the petri dish.

COMPARATIVE EXAMPLE 2 Evaluation of Antibacterial Properties (4)

[0103] The antibacterial properties were evaluated in the same manner as in Example 7 (i.e., without ultraviolet irradiation), except that sample 1 was replaced with comparative sample 1 (silver was not contained). After the culturing ended, 240 colonies of E. Coli were observed in the petri dish.

[0104] Table 5 collectively shows the results of Examples 6 and 7 and Comparative Examples 1 and 2. Sample 1 contains silver, which exhibits antibacterial properties. Therefore, sufficient antibacterial properties are also exhibited, even if ultraviolet irradiation is not performed. Therefore, when sample 1 is used with a photocatalytic filter device, it is not necessary to continuously operate the apparatus in order to prevent growth of microorganisms (e.g. bacteria). Consequently, operating costs can be reduced. TABLE 5 UV ray Antibacterial properties Sample irradiation (number of colonies) Example 6 Sample 1 (silver 30 min. 0 contained) Example 7 Sample 1 (silver blocked 0 contained) Compar- Comparative Sample 1 30 min. 0 ative (silver not contained) Example 1 Compar- Comparative Sample 1 blocked 240 ative (silver not contained) Example 2 

1. A photocatalyst composition comprising: a porous substrate having a surface, a photocatalyst layer disposed on the surface of the porous substrate, the photocatalyst layer primarily containing at least one photocatalytic material and having a surface, and at least one metal disposed predominantly on or near the surface of the photocatalyst layer.
 2. A photocatalyst composition according to claim 1, wherein the at least one metal is selected from the group consisting of silver (Ag), gold (Au), platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), copper (Cu) and nickel (Ni).
 3. A photocatalyst composition according to claim 1, wherein at least 80 wt % of the metal disposed on or near the surface of the photocatalyst layer is disposed on the surface of the photocatalyst layer.
 4. A pholocatalyst composition according to claim 1, wherein the metal comprises metal particles having an average particle size of between 1-100 nm.
 5. A photocatalyst composition according to claim 4, wherein at least 70% by number of the metal particles disposed on or near the surface of the photocatalyst layer are disposed on the surface of the photocatalyst layer.
 6. A photocatalyst composition according to claim 1, wherein the porous substrate is a ceramic having a three-dimensional network structure.
 7. A photocatalyst composition according to claim 6, wherein an average diameter of a backbone of the ceramic is between about 100 μm to 1000 μm.
 8. A photocatalyst composition according to claim 6, wherein the porous ceramic substrate exhibits a light transmissivity of at least 10% at a thickness of 5 mm.
 9. A photocatalyst composition according to claim 6, further having the following properties: (1) porosity of between about 65% to 99%, (2) bulk density of about 0.05 g/cm³ to 60 g/cm³, and (3) between 10-30 cells per 25 mm.
 10. A photocatalytic filter device comprising: the photocatalyst composition according to claim 1 and a frame supporting the photocatalyst composition.
 11. A photocatalytic filter device according to claim 10, further comprising a light source disposed proximally to the photocatalyst composition and emitting light that activates the photocatalytic material.
 12. A method for making a photocatalyst composition comprising: disposing a photocatalyst layer substantially comprising a photocatalytic material on a surface of a porous substrate and disposing at least one metal predominantly on a surface of the photocatalyst layer, the metal increasing the photocatalytic activity of the photocatalytic material.
 13. A method according to claim 12, wherein the metal disposing step further comprises contacting the photocatalyst layer with a solution containing ions the metal, and irradiating the photocatalyst layer with light having a wavelength that activates the photocatalytic material, thereby reducing and depositing the metal on the surface of the photocatalyst layer.
 14. A method according to claim 13, wherein the at least one metal is selected from the group consisting of silver (Ag), gold (Au), platinum (Pt), palladium (Pd), ruthenium (Ru), rhodium (Rh), copper (Cu) and nickel Ni).
 15. A method according to claim 12, further comprising preparing a ceramic porous structure having a three-dimensional network structure before the photocatalyst layer disposing step.
 16. A method according to claim 15, wherein an average diameter of a backbone of the ceramic is between about 100 μm and 1000 m.
 17. A method according to claim 16, further comprising attaching ceramic particles to a surface of the backbone of the ceramic.
 18. A method according to claim 15, wherein the prepared ceramic porous substrate satisfies at least one of the following conditions: (1) porosity of between about 65% to 99%, (2) bulk density of about 0.05 g/cm³ to 60 g/cm³, and (3) between 10-30 cells per 25 mm.
 19. A method according to claim 15, wherein the photocatalyst layer disposing step is performed such that light transmissivity at a thickness of 5 mm is at least 10%.
 20. A composition of matter comprising: a ceramic porous support having three-dimensional network structure with a backbone having an average diameter of between about 100-1000 μm, a photocatalyst material disposed on the backbone of the ceramic porous support, wherein the ceramic porous support having the photocatalyst material disposed thereon exhibits a light transmissivity of at least 10% at a thickness of 5 mm, and metal particles disposed substantially only on an outer surface of the photocatalyst material, wherein the metal particles include at least one metal selected from the group consisting of Ag, Au, Pt, Pd, Ru, Rh, Cu and Ni and the metal particles have an average particle size of between about 1-100 nm.
 21. A composition of matter according to claim 20, wherein at least 80 wt % of the metal disposed on or near the surface of the photocatalyst layer is disposed on the surface of the photocatalyst layer and the ratio of metal particles to photocatalyst material is between about 0.0005 and 5 wt %.
 22. A composition of matter according to claim 21, wherein the metal particles include at least one of Ag or Cu.
 23. A composition of matter according to claim 22, wherein the composition of matter exhibits the following properties: (1) porosity of between about 65% to 99%, (2) bulk density of about 0.05 to 60 g/cm³, and (3) between 10-30 cells per 25 mm.
 24. A photocatalytic filter device comprising: the composition of matter according to claim 23, a frame supporting the composition of matter and a light source disposed proximally to the composition of matter according to claim 23 and emitting light having a wavelength that activates the photocatalytic material. 